Pitch roof extn - vent above or below membrane?

[billm]

Building a single storey kitchen extension and getting close to putting the membrane on the rafters. I am putting 1.5m strip vents over the fascias and will be using felt support trays. Do I route the ventilation above or below the membrane? I have asked 2 roofers and got 2 different answers.

 

[ajstoneservices]

Below.

 

[^woody^]

The idea is that they ventilate the roof space.

But they are not needed if you use certain membranes

 

[billm]

The idea is that they ventilate the roof space.

But they are not needed if you use certain membranes

Thanks
which class of membranes?

The 1st roofer said ventilate above the membrane because the condensation forms on the underside of the roof tile and drops back onto the membrane.

The 2nd roofer said to ventilate under the membrane because it was in the cold space so condensation would form on its underside

 

[^woody^]

Look for an "air open" permeable membrane, as standard permeable membranes are not performing as well as expected and experiencing condensation on the underside in the loft in don't circumstances. Check for a BBA certificate that will normally state that the membrane does not need additional vents, and this will be what to refer the building inspector to.

Your first roofer, mat not be a roofer.

 

[billm]

Thanks Woody. I'll look out for that type of membrane. Does that mean I won't need an abutment vent where the roof joins the house wall ?

 

[freddiemercurystwin]

Correct. BTW your first'roofer' is clueless. I wouldn't let him anywhere near my roof. Can't remember the last time I saw a roof being done with a non breathable membrane.

 

[billm]

Correct. BTW your first'roofer' is clueless. I wouldn't let him anywhere near my roof. Can't remember the last time I saw a roof being done with a non breathable membrane.

Abutment vent: thanks.

No roofer #1 said to use a breathable membrane which would let the moisture through but it would then condense on the cold tiles so that's why he said to put the ventilation above the membrane, like drawing 101 here I guess:
http://www.fastrackcad.com/CADFrame...=0&Level0=002$FIBRE+CEMENT+SLATE+APPLICATIONS

 

[^woody^]

Does that mean I won't need an abutment vent

Yes, as long as you use one of the AO membranes in accordance with their installation instructions or BBA certificate.

This should not be a problem with your inspector, but you might want to run it past him first, as he might not be up to date.

 

[billm]

Thanks Woody, explained in this video too, cheers.